Do not duplicate attachments when decrypting OpenPGP mails

Previously the code walked over the decrypted message n and added all
the attachments. But since mime mails are hierarchical and all
attachments are already contained in n, any attachments were
duplicated in the resulting mail object m. Fix this by just attaching
n to m.

Signed-off-by: Justus Winter <4winter@informatik.uni-hamburg.de>